How to remove or move client from public group???

[Nokall]

Hello

When costumer register in site by default go to public group , in admin area it´s not possible to remove or move from public group to a certain group that i whant.
Discount group it´s gived by default to all site
Example : Retail 20% , Buy more 30% when i give 30% costumer have 30% in all categorys because off public group if can remove or move costumer after regist from public group and move to certain group we can have some categorys don´t give discount.

Any idea or any tip ?


Thanks